As light emitting diodes (LEDs) are increasingly used for illumination purposes, in particular, as a substitute for light bulbs, adequate driver circuitry has been subject to research and development in recent times. Inter alia, one desired object of such development efforts is to increase efficiency, that is to reduce power dissipation in the driver circuitry. Other development goals include, an increased flexibility of use and low costs.
One LED based illumination device usually includes a series circuit of a plurality of LEDs, a so-called LED chain. As LEDs usually have to be driven by a defined current, each LED in a LED chain is supplied with a fixed (not necessarily the same for all the LED chains) current. The supply voltage, necessary for driving the LED chain depends on the number of LEDs present in the chains because the forward voltages of each of the single LEDs sum up to the required supply voltage of the LED chain. It is known that the forward voltages may heavily vary due to temperature variations, variances in the manufacturing process and other parameters. As a consequence, the supply voltage necessary to provide a desired load current may vary and the driver circuitry used to drive the LED chain should consider such variations.
In order to guarantee a defined brightness and color hue, the supply current of the LED chain is to be monitored and regulated so as to stay at a predefined reference level or at least stay within a small interval around the reference level. Linear current regulators are commonly used for the described purpose of supplying a defined current to the LEDs. However, the driver circuit has to be designed for the worst case, that is for the maximum possible supply voltage which might occur across the LED chain. Such a design entails undesirably high losses in the above-mentioned current regulators.